Flame-Retardant Treatments of Cellulose and Their Influence on the Mechanism of Cellulose Pyrolysis

Abstract

Cellulose, either as a major component in wood or as the prime textile fiber cotton, is most frequently implicated in fire, causing injuries and fatalities [1]. When ignited, cellulose undergoes thermal degradation, form-ing combustible volatile compounds which become involved in the propaga-tion of fire. Fortunately cellulose has a chemical composition which makes it easily amenable to interactive flame-retardant treatments. Because flam-mability is a relative rather than an absolute concept, there are no truly flame-retardant fabrics, and the best that can be attained is some given level of flame resistance. Barker and Drews [2] proposed that with cellulose, the problem of fire can be described as two distinct phenomena, glowing and flaming, which present different potential hazards and should be ap-proached in different ways. Glowing is a direct oxidation of solid cellulose or its degradation products. It is generally a slow combustion and is of great concern for only specific items, such as carpets, upholstery, mattresses, and insulation. Flaming combustion, on the other hand, is a complex process involving both solid and gas phases, and may be modeled as a cyclic pro-cess. In the initial stages of burning, heat is supplied to cellulose, initiating endothermic degradation reactions in which large polymer molecules are broken into smaller, volatile fuel compounds. The pyrolysis products dif-fuse to the surface and mix with oxygen from the air so that combustion may take place. This combustion is exothermic, and the heat thus liberated is partially transferred back to fiber surfaces to continue polymer pyrolysis, maintaining a continuous supply of gaseous fuel for further propagation. This process is shown schematically in Fig. 1 [3, 4]. Early attempts to explain the mechanism of flame-retardant action of